Method of treating fibers



States tent 3,lll,3'fi3' Patented Nov. 19, 1963 hire 3,111,363 METHOD OF TREATING FIBERS Akihumi Nasuno, Kurashiki, Japan, assignor of threefcurths to Kurashiki Rayon (30., Ltd, Kurashilii-shi, Japan, a corporation of .lapan, and one-fourth to Air Reduction Company, Incorporated, New York, N .Y., a corporation of New York No Erawing. Filed Feb. 23, 1961, Ser. No. 90,927 Claims priority, application Japan Feb. 23, 1959 5 Claims. (Cl. 8115.5)

This invention relates to a process of imparting improved properties to fibers formed from polyvinyl alcohol and is more particularly concerned with a process for treating polyvinyl alcohol fibers to increase their dyeability.

It has long been known that, in comparison with cellulose fibers or other natural fibers, fibers of polyvinyl alcohol generally have a low rate of dye penetration under conventional dyeing conditions so that the time required for dyeing polyvinyl alcohol fibers is much longer. in addition, when compared with cellulose fibers or other natural fibers dyed at identical rates of dye absorption, the apparent deepness of the dyed color, is low in the case of polyvinyl alcohol fibers and the color is relatively less bright.

It is an object of this invention to provide a method for improving the dyeing characteristics of polyvinyl alcohol.

I have discovered that by stripping the surface of the polyvinyl alcohol fibers, to remove a small percent by weight of the fiber from its exterior surface inwardly, the dyeing properties can be improved and the apparent deepness of the dyed color can be raised. The reason for the improvement in the fiber by such stripping is not fully understood, but it is presumed that a drop in the rate of dye penetration, or in the deepness of the dyed color, is caused by the presence of a very thin and fine-grained surface film which is produced upon the fiber in the process of manufacture and which is quite different from the interior of the fiber. In accordance with this invention, such stripping to remove the surface film is effected by esterifyiug the exterior of the fiber and then removing the esterified portion by means of solvents, and saponifying any ester radicals which may remain. To be elfective to produce the desired improvement in dyeing characteristics, stripping is carried out to remove more than 6.1% by weight, preferably at least 2% by weight, of the original fiber, but not more than about to by weight. if the percentage of stripping is increased beyond 20%, not only will the weight loss increase but the strength and other properties of the fiber will tend to deteriorate. Moreover, if stripping is effected beyond 20%, I have discovered that, surprisingly, instead of increasing further in dyeability, the fibers tend to decrease in dyeability, which is, needless to say, very disadvantageous. Hence it is very important that stripping be limited to the removal of the surface film which is considered to have an adverse effect on dyeing. In other words, the percentage of stripping must be restricted to a maximum or" 20% by wei ht based on the weight of the fiber treated, but it is necessary to carry out stripping to an extent of more than 0.1% by weight, preferably at least 2% by weight, in order to achieve the desired increase in dyeing properties of the present invention.

In accordance with the invention, stripping is etected by esterification, viz. by treating the fibers with an esterifying agent such as acetic acid, an acid anhydride, e.g. acetic anhydride, or an acyl chloride such as benzoyl chloride. The esterifying agent can be suitably employed in vapor form but it is also advantageously used insolution in an inert solvent which, in the case of an acid anhydride, or in the case of acetic acid, has been acidified,

g. with sulfuric acid, the time of treatment being selected in proportion to the extent of stripping to be effected. When an acyl chloride is employed as the esterifying agent, the treating solution also suitably includes a hydrogen chloride acceptor, viz. a tertiary amino compound, such as pyridine.

The present invention will be more fully understood by reference to the following series of comparative tests wherein treatment was effected upon polyvinyl alcohol fibers produced by wet-spinning in conventional manner, heat-treatment for 5 minutes in hot air at 215 C., and acetalization an aqeuous solution containing formaldehyde (5%), sulfuric acid (15%) and sodium sulfate (15%) at 69 C. for 30 minutes, all percentages being by weight, to a degree of acetalization, i.e. formalization, of 32 mol percent.

These fibers, after drying at 60 C. under vacuum were then acetylated by immersion in a solution composed of acetic anhydride (6.5 parts by weight), concentrated sulfuric acid (1.5 parts by weight) and benzene (19.4 parts by weight) at 40 C. for varying periods of time. Thus, difierent samples of the fibers were treated for 1 minute, 2 minutes, 3 minutes, 5 minutes, and 10 minutes. The thus acetylated products were then extracted with benzene or methanol, and any residual acetyl groups were saponified with 21 methanol solution of caustic soda, followed by washing, pressing, and drying. The thus-treated formalized polyvinyl alcohol fiber samples were found to have been stripped by 2.5%, 5.0%, 7.7%, 12.2%, and 24.5%, respectively, in proportion to the times of treatment. In similar manner, others of these fibers were acetylated in a solution composed of 6.5 parts by weight of acetic acid, 1.8 parts by weight of concentrated sulfuric acid, and 19.4 parts by weight of benzene at 40 C. for 12 minute, 1 minute, 2 minutes, 3 minutes, 5 minutes and 10 minutes. The acetylated fibers were then extracted and saponified as described above and they Were found to have been stripped by 0.2%, 2.5%. 5.0%, 7.7%, 12.2%, and 24.5% respectively, in proportion to the times of treatment.

When the cross-sections of the treated fibers were observed in an optical microscope, it was ascertained that the stripping had advanced from the outermost surface inwardly in proportion to the respective tirnes of treatment. When the external surfaces or" the stripped fibers were examined by means of an electron microscope, it was found that the uneven longitudinal stripes inherent in polyvinyl alcohol fibers had disappeared, md a relatively smooth external surface was observed. These observations confirmed that stripping of the outermost surface of the fibers had been accomplished. The following table shows the various changes in several properties of these fibers:

TABLE 1 Degree Hot water- Time of Extent Dry resistance Treatof Dry Elon- Formal- (Shrinkment Strip- Denier Strength gation ization age in (Min ping (g./d.) (per- (mol hot water (percent) cent) percent) at 100 0.,

(percent) 1 The values indicated above were found to be the same in the case of the acetic-acid treated fibers and in the case of the acctic-anhydride treated fibers.

The stripped fiber samples shown in Table 1 were then dyed with Congo red in various concentrations in dye baths containing sodium sulfate by weight), with a bath ratio of 50:1, at 80 C., for 2 hours, whereby fibers of varying degrees of dye absorption were produced and the dye absorption and the deepness of the dyed color were compared, the results being shown in the following table:

Example 1 Polyvinyl alcohol fibers obtained by conventional wetspinning were subjected to dry heat-treatment in an air bath at 220 C. for 5 minutes, and were then acetalized to the extent of 33.5 mol percent in a formalization bath containing formaldehyde (5%), sulfuric acid (15%), and sodium sulfate (15%), following which the fibers were dried. The thus acetalized fibers were then immersed in a solution consisting of acetic anhydride (6.5 parts), concentrated sulfuric acid (1.8 part) and benzene (19.4 parts) at room temperature for 2. minutes, and then thoroughly washed with benzene, although methanol could also have been effectively used for this purpose, until acetylated polyvinyl formal no longer was present in the wash liquid. The residual acetyl groups in the fibers were then saponified by means of a conventional saponification medium consisting of a methanol solution of caustic soda.

The amount of stripping of the thus-treated fibers was 4.8%, and no measurable change was observed in the mechanical properties of these stripped fibers when compared with the fibers before the stripping treatment. When the stripped fibers were dyed with various dyestuffs under conventional dyeing conditions, the treated TABLE 2 Stripped Fiber-Percent Stripped Dye Absorption (mg. of dyestnfi per g. of

fiber):

Dye concentration in the dye bath,

percent by weight stufi, a 7.0 8.8 9.0 12.0 10.5 9.2 8.0 (K/S): Dye Absorption, 10 mgJg 6.5 7.4 7.5 9.5 8.3 7.8 6.5 Brightness (Stimulus values in percent by CIE chromatic specification of dyed fiber 0fK/S=10) 53 55 56 63 57 55 51 1 The values indicated above were found to be the same in the case of the acetic-acid treated fibers and in the ease of the acetic-anhydride treated fibers.

The ratio K/S is obtained from the Wave-length (520 mu) at maximum absorption of the curve of the refraction coefficient of the dyed fiber by the following formula:

'(Kubelka Munk formula) From results such as shown in Table 2, it is observed that when polyvinyl alcohol fibers are stripped of their outermost surface by the process of this invention, the best dyeability is exhibited when the fiber is stripped to the extent of a few percentage values, and when the stripping exceeds not only is there a drop in the fiber strength but a drop in dyeability also occurs.

It is also noted that the dyeing rate is markedly improved in the case of stripped fibers. On the basis of these observations, it is thought that a thin but relatively close-grained skin layer (a few percent by weight of the total fiber) is present on the external surface of the polyvinyl alcohol fiber, that this layer has an adverse effect upon dye penetration, and the dyeing characteristics of the fiber to be dyed, and that by removing this skin layer, its adverse effect is eliminated.

The invention will be further understood from the following specific examples of practical application. However, it will be understood that these examples are not to be construed as limiting the scope of the present invention in any manner. In these examples, all parts are by Weight, unless otherwise indicated.

fibers showed higher rates of dye absorption compared with the untreated fibers. Moreover, under identical dyeing conditions, and even at an identical rate of dye absorption, the treated fibers had a higher color depth, and the dyed shades were much brighter.

When this example was repeated but using an acetylating solution containing 6.5 parts by weight of acetic acid instead of 6.5 parts of acetic anhydride, 9. corresponding stripping of the fibers was obtained and fibers having comparable dyeing characteristics were produced.

Example 2 Polyvinyl alcohol fibers produced by conventional dryspinm'ng were heat-treated in an air bath at 210 C. for 10 seconds, and were then acetalized to the extent of 35 mol percent in a formalizing bath consisting of formaldehyde (5%) and sulfuric acid (15%). The acetalized polyvinyl alcohol fibers were then continuously treated in a solution composed of acetic anhydride (5 parts), concentrated sulfuric acid (2.5 parts) and ethylene chloride (20 parts) at 50 C. for 30 seconds, with subsequent Washing with methanol until acetyla-ted polyvinyl formal was no longer present in the wash liquid. The fibers Were thus stripped to the extent of 3.8%. There were no changes in the mechanical strength and hot water-resistance of the fibers when these properties were compared before and after the stripping treatment. When dyed with various types of dyes in conventional manner, the stripped fibers exhibited excellent dyeability as in the case of the fibers of Example 1.

When this example was repeated but using an acetylating solution containing parts by weight of acetic acid instead of 5 parts of acetic anhydride, a corresponding stripping of the fibers was obtained and fibers having comparable dyeing characteristics were produced.

Example 3 Polyvinyl fibers obtained by conventional Wet-spinning and formalized as in Example 1 were dipped in a solution of sulfuric acid and the fibers were then exposed to acetic anhydride vapors for about thirty minutes, and washed thoroughly with methanol as in the preceding examples. The stripped fibers thus obtained showed a weight loss of about 8% as compared with the original fibers. When these treated fibers were dyed with various types of dyes in conventional manner, they exhibited excellent dyeing characteristics.

Example 4 After polyvinyl alcohol fibers produced by conventional dry-spinning had been heat-treated in an air bath at 220 C. for 10 seconds, they were immersed in a solution consisting of benzoyl chloride (6.4 parts), pyridine (7.2 parts), and benzene (2O par-ts) at room temperature for 5 minutes, thoroughly washed in a benzene solution, and then saponified with a methanol solution of caustic soda. The treated fibers were formalized to a 38 mol percent degree of acetalization in a formalizing bath composed of formaldehyde (5%), sulfuric acid and sodium sulfate The thus-formalized fibers were found to have been stripped to the extent of about 5.5% as compared with the original fibers. When these stripped fibers were conventionally dyed with various types of dyes, the dyed stripped fibers obtained exhibited excellent dyeability as compared with the untreated fibers.

Example 5 Formalized polyvinyl fibers obtained by Wet spinning as described in Example 1 were dried, and they were treated at room temperature for a minute in a solution composed of 6.5 parts of glacial acetic acid, 0.9 part of concentrated sulfuric acid, and 19.4 parts of benzene. The fibers were then thoroughly washed with methanol until acetylated polyvinyl formal was no longer present in the Wash liquid, and the remaining acetyl groups were saponified, using a methanol solution of caustic soda. The amount of stripping of the thus treated fibers was 0.2%. The mechanical properties and the hot water resistance of the fibers were not altered by the specified treatment, and when the fibers were dyed with various dyes in accordance with the conventional procedures, excellent dyeing results were obtained, very similar to those shown by the fibers of Example 1.

Example 6 Polyvinyl alcohol fibers obtained by conventional Wet spinning were heat-treated for 5 minutes in an air bath at 220 C., were acetalized to the extent of 23 mol percent in an aqueous solution containing 2% of benzaldehyde, 10% of sulfuric acid, and 40% of methanol, and were dried. The outer skin of the fibers was then stripped in the manner described in Example 1. The extent of stripping of the treated fibers was 5.8%, and the mechanical properties showed almost no change. When the fibers were dyed with various dyes in conventional manner, excellent dyeing results were obtained comparable to those found with the fibers of Example 1.

As will be seen from the foregoing examples, vaporizable esterifying agents such as acetic anhydride and acetic acid may be applied to the fiber in vapor form, the fiber previously being brought into contact with a mineral acid catalyst, e.g. sulfuric acid. However, when the esterifying agent is employed in the form of a solution, any inert solvent is suitably used such as an aromatic hydrocarbon, e.-g. benzene, an alkyl alcohol, such as 6 methanol, or a chlorinated hydrocarbon, such as ethylene chloride. Any of these solvents are also suitably used for the subsequent Washing operation. Similarly, the esterifying agent may be any acid anhydride or any acyl chloride but lower-alkyl acid anhydrides such as acetic anhydride, and aromatic acyl chlorides such as acetyl chloride and benzoyl chloride are preferred. It will be understood, however, that other esterifying agents of this type may be employed such as acetyl chloride, phthalic anhydride, succinic anhydride, propionyl chloride, and the like. In general, unless otherwise indicated, the conditions employed are those conventionally for the esterification of compounds containing hydroxyl groups.

The preparation and treatment of the fiber prior to the esterification and subsequent treatments of the process described above form no part of the present invention and the process of this invention is applicable to all types of fibers derived from polyvinyl alcohol and produced in conventional manner and subjected to conventional treatments and processing operations, such as heattreatment, acetalization, and the like. Such fibers are conventionally spun from spinning solutions by known Wet and dry spinning techniques. Spinning techniques are described, for example, in Cline et al. Patent 2,610,- 360 and Osugi et al. US. Patent 2,906,594. Typical processes of treating polyvinyl alcohol fibers are also described in the foregoing patents, as well as in Suyama et al. US. Patent 2,890,927. With respect to known saponification techniques which are employed in accordance with the present invention following the esterification reaction, reference is made to the saponification procedures described, for example, in Bristol US, Patent 2,700,035, Waugh et al. US. Patent 2,642,419", and Germain Ufi. Patent 2,643,994.

Thus, the conditions and the relative relationships set forth in the examples are those preferred in carrying out the process of this invention, but it will be understood that other conditions and relationships may be used with in the scope of the invention. In general, unless otherwise indicated, conventional operations and techniques are suitably employed.

It will also be understood that various changes and modifications in addition to those indicated above may be made in the embodiments herein described without departing from the scope of the invention as defined in the appended claims. It is intended, therefore, that all matter contained in the foregoing description shall be interpreted as illustrative only and not as limitative of the invention.

This is a continuation-in-part of my copending application entitled Treatment of Fibers, Serial No. 61,835, filed October 11, 1960.

I claim:

1. A process for treating polyvinyl alcohol fibers to increase their dyeability which comprises the steps of subjecting said fibers to the action of an esterifying agent to esterify the exterior portion of said fibers, and washing the thus-treated fibers with a solvent to remove the esterified portion to effect a reduction in the Weight of the original fibers of more than 0.1% but at most 20%.

2. A process for treating polyvinyl alcohol fibers to increase their dyeability which comprises the steps of subjecting said fibers to the action of an esterifying agent selected from the group consisting of acetic acid, acyl chlorides and acid anhydrides to esterify the exterior portion of said fibers, and washing the thus-treated fibers with a solvent to remove the esterified portion to efiect a reduction in the weight of the original fibers of more than 0.1% but at most 20%.

3. A process for treating polyvinyl alcohol fibers to increase their dyeability which comprises the steps of subjecting said fibers to the action of an esterifying agent in vapor form to esterify the exterior portion of said fibers, and washing the thus-treated fibers with a solvent to remove the esterified portion to effect a reduction in the weight of the original fibers of more than 0.1% but at most 20%.

4. A process for treating polyvinyl alcohol fibers to increase their dyeability which comprises the steps of subjecting said fibers to the action of an esterifying agent in solution to esterify the exterior portion of said fibers, and Washing the thus-treated fibers with a solvent to remove the esterified portion to effect a reduction in the weight of the original fibers of more than 0.1% but at most 20%.

5. Polyvinyl alcohol fibers of increased -dyeability produced by subjecting polyvinyl alcohol fibers to the action of an esterifying agent to esterify the exterior portion of said fibers, and washing the thus-treated fibers with a solvent to remove the esterified portion to efiect a reduction in the Weight of the original fibers of mor than 0.1% but at most 20%.

References Cited in the file of this patent UNITED STATES PATENTS Dahle Aug. 24, 1943 Wilson et a1 Dec. 24, 1946 Dreyfus Sept. 9, 1947 Clime et a1. Sept. 16, 1952 Waugh et a1 June 16, 1953 Germain June 30, 1953 Stanin et a1 Aug. 18, 1953 Bristol Jan. 18, 1955 Cline June 5, 1956 Suyama et a1. Sept. 9, 1959 Osugi et a1. Mar. 28, 1961 

1. A PROCESS FOR TREATING POLYVINYL ALCOHOL FIBERS TO INCREASE THEIR DYEABILITY WHICH COMPRISES THE STEPS OF SUBJECTING SAID FIBERS TO THE ACTION OF AN ESTERIFYING AGENT TO ESTERIFY THE EXTERIOR PORTION OF SAID FIBERS, AND WASHING THE THUS-TREATED FIBERS WITH A SOLVENT TO REMOVE THE ESTERIFIED PORTION TO EFFECT A REDUCTION IN THE WEIGHT OF THE ORIGINAL FIBERS OF MORE THAN 0.1% BUT AT MOST 20%. 